Radio system



Jan. 23, 1940. A, A THOMAS' 2,188,165

RADIO SYSTEM Filed May 14, 1957 Patented Jan. 23, 1940 UNITED sTATEs PATENT oFFicr-zl 2,188,165 Ramo SYSTEM Delaware Application May l14, 1937, serial No. 142,546 z claims. (c1. ass- 52) My invention relates to the art of radio broadcasting, and its object is to provide means for ascertaining the number of receivers tuned in on a given program. In this way the commercial sponsor of a broadcast program will have deflnite information on the publics attitude toward his program, and that, from the sponsors viewpoint, is the most important thing about any piece of advertising broadcast. Herein `liesthe practical importance of my invention.

As everybody knows, radio advertising is highly expensive and its value to the advertiser depends upon the number of listeners-in. 'I'hat is why station announcers so frequently urge their unseen audience to send-in their opinions and suggestions concerning the program. Sometimes an advertiser ofie'rs a little gift to those who will write'for it. However, it is a known fact that only a negligible percentage of listeners-in take the trouble to respond by mail or telephone, so that the number of communications received by a station in reference to a given broadcast fur- -nishes no reliable basis for ascertaining the size of the invisible audience that heard the program.

It is the purpose of "this invention tolfurnish a radioV advertiser with accurate information regarding the number of receivers tuned in on his program, so that he can determine how well it is going with the public and to improve it if he nds that his audience is dwindling. In carrying out the principles of my invention, I provide each radio receiver with a device capable of transmitting a certain-signal adapted to be received only by the station for which the receiver is in tune. These transmitting devices of the receivers are controlled entirely by the operator at the tuned-in station and without the knowledge p of the listeners-in, so that no interference with the regular broadcast is possible. Detector mech- 4'0 anism at the broadcasting station receives a certain amount of electric energy from eachoperat- 5 tion will be understood from a'description of the accompanying drawing, in which Fig. 1 shows diagrammatically the apparatus at a broadcasting station for sending out certain signals to tuned-in receivers and receiving the response signals from such receivers:

` Fig. 2 is a diagrammatic illustration of the ymechanism associated with a radio receiver for receiving the special signal sent out by atunedin station and for transmitting the response signal to that station; and 6 Fig. 3 indicates in a diagrammatic way a plurality of broadcasting stations and a plurality oi receivers, these stations and receivers being equipped. with the-.special transmitting and re-l ceiving apparatus shown in Figs. 1 and 2. 10

Before describing the drawing I want to explain that I have made it as simple as possible for clearness. Such app atusas is well known I have indicated merely in schematic outline, and only such parts of standard outts used in broad- 15 casting stations and radio receivers appear in the ldrawing as are necessary to describe my invention.

'I'he various pieces of apparatus illustrated in Fig. 1 are parts of the operative equipment of a 20 broadcasting station employing my invention and are placed in suitable locations. 'I'he transmitting antenna circuit is represented by a highfrequency generator I0, a tuning coil l2, an aerial I3, 'and combined' amplifying and modulating 25 mechanism M. The circuit of the broadcasting microphone I4 includes a coil I5 arranged in transformer relation to the modulating input coil I6 of mechanism AM,u whereby the electric impulses produced by the microphone are imposed 30 in amplied form upon the` carrier frequency transmitted by the antenna |3. The above mentioned parts are standard equipment requiring no detailed description of structure and operation.

Switch mechanism W, Aconveniently located, 35` .V comprises a plurality'of switches represented by the pairs of contact .members I'|-`I8, l9-20, 2|22. 23'-24, 25-26 and 21-28. The ilrst member of each pair is a movable arm supported at one end, and the other member is a fixed con- 40 tact arranged to be engaged by the free end of the' arm for closing a circuit. The three switch arms I1-I 9-21 are mechanically connected by an insulating cross rrod or pin 29, so that they always move' in unison; Likewise, 4the three '45 switch arms 23-25,-' 2'|Y are :loinedby an insulating connection 30 for simultaneous operation. The normal positions of the six switch arms are such that the four switches |1|8, 2i 22,25-26 and 21-28 are normally open, while the other 50 -two switches I9--20 and 23-24 are normally closed. 'Ijhe movable switch arms may be spring blades held in normal position by inherent tension, or I may use separaterestoring springs 3| and 32, the rst lbeing connected to switch arm 2| andthe other to switch arm 25. 'Ihe contracting spring 3I normally holds the arm I3 against the fixed contact 20 and keeps the two connected arms I1 and 2I awayfrom their respective con- 5 tacts I8 and 22. The other coil spring 32 normally presses the switch arm 23 against the fixed contact 24, while the associated arms 25 and 21 are kept away from their respective contacts 25v and 28. The iixed contacts 20 and 24 also constitute stops for limiting the movement of the `switch arms under the action of springs 3l and The various switches of mechanism W are mounted on a suitable insulating panel (not l5 shown) which has a two-way groove consisting of a down passage 33 and an up or return passage 34. A switch-actuating roller 35 is arranged to operate in groove 33-34, the down movement of the roller being in passage 33 and the up movement in passage 34. This two-way travel of roller 35 is assured by spring latches 36 and 31, which normally extend into the end portions of passages33 and 34 respectively,o so that these latchesA -are cammed out of the way by the roller at the end of its downward and upward movement. The. cammed or inoperative positions of the latches 36 and 31 are indicated by thebdotted outlines 36' and 31.- When the roller 35 is in normal position at the top of the groove, the latch 31 compels the roller to travel down through passage 33 till it reaches-its lowest position 35 after camming the latch 36 out of the way. This latch now closes the passage 33 and compels the roller to travel up through the passage 34 till it 35 reaches its normal position at the top .of the groove after camming thelatch 31 aside.

The roller 35 is actuated down and up the two-waygroove 33-34 by a lever 38 pivoted at 39 Mintermediate its ends. One end of lever 38 is o connected to roller 35 by a link 40, which is indicated by a dotted line so asnot to obscure the drawing, it being understood that this line rep- .resents .any practical connection between roller and lever. 'I'he top ahdbottom ends of groove 33-34 form stops for roller 35 and lever 38,

which is normally held in uppermost position by a contracting coil spring 4I. The lever'38 can be operated by hand like a pivoted switch arm, but I prefer to operate it electrically so as to insure l o uniformity of time at every operation, ttor a reason that will become apparent later on. Ahaligear 42 xed on lever 38 around its pivot meshes with a half-gear 43 mounted on shaft 44, which also carries a pulley 45 connected by a 'cord or` chain 46 to a magnetic core or plunger 41 of a solenoid coil `48. When this coil is energized, the plunger 41 is pulled rdown and rocks the lever 38 to position 38', whereby the roller 35 is carried down the angular passage 33 to position 35.

00 When the coil 48 is de-energized, the tensined spring 4|- returns the lever 38 to normal position and the roller 3511s carried up the angular i passage 34 to the top of the groove. .The broadcasting station vhas a signal general5 tor F of suitable construction adapted to produce a special' signal of predetermined character or frequency.' The signal generator F may be an altcrnaton-a vacuum tube oscillator, an interrupter mechanism, 'or any other piece of vappa TiyratuS' designed tov produce electric impulses oi certain .frequency or any kind of electric signal of such character as to be outside the regular broadcasting range. For thefpurposes of this description we may assume that the signal pro- 'll duced by generator F consists of electric oscillations of a specific frequency which does not ini terfere with the broadcasting operations of any station and which is adapted to control certain electric devices in radio receivers, as will be fully explained in connection witlr Fig. 2. So for the` present I need only say' that the signal frequencyv of generator F is impressed at the proper time -upon the input coil I6 of the amplier-modulator M. The output leads 49-50 of generator F are connected, respectively, to one side of coil I5 and to contact I8, which isnormally open, as previously mentioned, so thatthe signal generator is normally in an inoperative condition andhas no eiect on the broadcasting oi a program. The

other side of coil I5 is connected by a conductor` 5I to switch armI I1 and also to one side of the microphone I4 by a wire 52, which goes to conductor 5I. The other side of microphone I4 is connected by a wire 53 to switch arm I9, which i's normally closed, and the contact 28 is. connected by a wire 54 tolead 49. The conductor 55 of the antenna circuit goes to'the two switch arms 23 and 25, which are electrically connected at their supported ends by a metal piece 56. The conductor 55 of the antenna circuit goes to contact 23, and contact 26 .is grounded so as to connect with the grounded end of the antenna circuit.

The antenna coil I2 has inductively associated therewith a coil 51, which is part of the tuned input circuit of an electronic detector-amplifier A D. As will become clear from the description of Fie. 2, the function of mechanism' D is to re-` ceive (and amplify if necessary) a'certain response' signal transmitted by every receiver in tune with the broadcasting station of Fig. l. 'I'he output circuit 58 of the receiving mechanism l)V is energized in proportion to the total electric energy received by the input coil 51, and the resultant current in the output circuit operates a suitable measuring instrument indicated as a whole by E. As we are here dealing with highfrequency currents, the electric operative ,element roi! instrument E should be practically free koi! self-induction, and one type 4of such an -insti-uf ment is a hot-wire ammeter, which I have shown diagrammatically in its essential parts. A `ilne wire H (usuallyV of platinum, platinoid, or copper) is s pported at its ends in taut condition and conn ctd in the output circuit 58. The

conductor H is connected at or near its center* to o e end of a strong .thread or wire 59, which. is s' curedat the other end to an adjustable sup'- port 53a. The illar member 5941s connnected 'at the center to another thread or wire. 53', lwhich passes once or twice around a pulley 60 rilxed on a rotary spindle 60'. The end of the lar element 53' is connected to a'contracting coil spring 5I, which holds the electric `wire H and the lar connections 53--59' always taut.. An arm 62 iixed on the -spindleill terminates in` a yoke 62' which carries a thin wire or strong thread 52a wound once or twice around la pulley' 83 iixedon a spindle or arbor. 63'. The lar connections 53' and 82a are always taut enough Yto operate their respective pulleys 50 and 63. A

pointer 54 nxed on arbor 63 moves over a scale S marked oiI to indicate the numbers of tuned- -in radio receivers or having equivalent notations.l The adjustable support 59a for wire 53 may be a thumbscrew or the likefor adjusting .the pointer 84 to normal zero position.

The operation of measuring instrument "E' is like this: The electric energy passing through the output circuit 5 8 heats the tensioned wire ccv H, which thereby expands and causes the spring 6| to rotate the arm 62, and this rotates the pointer 64. Due to the amplifying connections between the pulley shaft 68 and the pointer shaft 63', the small movements of shaft 89' are transmitted to the pointer 64 in greatly increased ratio, so that a large indicating scale is possible. I have not attempted to show the transmission connections between wire H and pointer 84 with mathematical accuracy, as that detail lies within the skill of the instrument maker. It will be clear that the angle through which the pointer 64 is turned depends upon the elongation of wire H due to the heating effect of the current passing through it, and that heating effect is strictly proportional to the quantity or ampere value oi the current. In other words, the movement of pointer 64 is governed by the amount of electric energy received lin coil 51, and the amount of* that energy, as will appear later, represents the number of receivers in tune for this particular station at a given moment.

The controlling wire H may be connected in series with the output conductors 58 to receive the full current now, or it may be shunted across an inductance 58' of known value so that only a certain fraction of the total current traverses the wire H. The scale S is, of course, calibrated accordingly. The moving parts of instrument E can be made very light, so as to have minimum inertia and friction, and as the wire'H has practically no self-induction, this instrument is well adapted for measuring high-frequency currents with accuracy. In some instances, depending upon the sensitivity of `instrument E, it may not be necessary to lnterpose a current amplier between thev input coil 51 and the output circuit 58 of theA detector mechanism D. The fllar connections 59, 59' and 62a should be of a material substantially unaiected by temperature and atmospheric conditions. If only sight indications of pointer 64 are required (as distinguished from recording), the scale S may have a movable index member 64' pushed along by the tip of ther pointer and adapted to remain in indicating position when the pointer returns to zero. This gives the observer time to read the scale at his convenience, even if the pointer is actuated only for a moment. On the other hand, the pointer 64 may have a recording tip for making marks on a properly graduated sheet or cylinder operated by clockwork, as is usual in recording instruments of variousv types. It is to be understood that, in the broader aspect of my invention, the instrument E represents any practical device operated or controlled by the amount of electric energy received in coil 51 ata given time and indicating or recording the equivalent oi'said energy in terms'of the total number of receivers tuned in at that time.

The tuned input circuit of detectorD includes two conductors 65-66, which are connected to the normally open switch` members 21-28, so

that this mechanism is normally in an inoperative condition and doesnot interfere witIVthe broadcasting operation of the station. The movable switch arm |1 is shaped to have a central portion |1' projecting across the down `passage 33, and the opposite switch arm 23 has a simllar portion 23' projecting across the return passage 34. Consequently, the cam section |1 is engaged by roller 35 in its down movement, and when the roller moves up it engages the cam section 23. The result is that "during the down travel of -roller 35 the normally open switches |1|8 and 42|-22 are closed and the normally closed switch |9-20 is opened. As the roller 35 moves up through passage 34, it opens the f normally closed switch 23-24 and closes the normally open switches 25--26 `and 21`-28. It is desirable to time the closed period of switch |1|8 which controls the signal generator F, and for that purpose I provide means for automatically holding the switch closed a predetermined period regardless of the length of time that the roller 35 engages the cam section |1' and independently of the manual control of coil 48. A simple arrangement for accomplishing that result comprises the following mechanism:

A rotary knob 61 (which represents any. suitable lnger piecel is mounted ona shaft 68 on which is lixed an arm 69 carrying a pivoted pawl 10. This pawl is plvoted at 1| to arm 69 and a light spring 12 normally holds the upper end o'f the pawl against the side of the arm. When the knob 61 is turned clockwise, the pawl 10 forms a rigid extension of arm 69. but during the return movement of the knob, the pawl is tree to swing clockwise about its pivot 1|. The rotary movements of knob 61 are limited by the iixed stops 13-14, and a contracting coil spring 15 normally holds the arm 69 against the stop 18. This spring also returns the knob to normal position when manually released. A switch arm 16 is normally held away from its contact 11 either by inherent tension or by a separate spring.

A cam or hump 16' on switch arm 16 is in the. path of pawl 10, so that the switch 16--11 is,

closed when the knob 61 is turned clockwise. During the return of knob 61, the end of pawl 18 rides idly over the hump916' because the pawl l is free to swing clockwise aboutits pivot 1I.

For convenience of illustration I have shown a separate battery 18 as the source of current for the solenoid coil 48, but in actual practice this coil will be connected to the regular feed mains of the station. One side of battery 18 goes to coil 48 and the other side to switch contact 11. A movable switch arm 19 (which may "be a spring linger) is normally in circuit-closing position against a xed contact 80, which is connected by Wire 8| to coil 48. The switch arm 16 carries an armature 82 arranged to be drawn to the xed magnetic core 83 of coil 84, which l is connected by wire 85 to switch `arm 16 and by wire 86 to switch arm 19. According to these connections,'the coils 48 and 84 are in series with the-(series switches 16-11 and 1er-so. As the switch 16-11 is normally open, the coils 48 and 84 remain inert even though the switch 19-80 is closed. Therefore, when the knob 61 is turned right.' the `switch 16-11 isclosed and `the coils 48 and 84 are energized. As previously explained,

the energized coil 48 operates the lever 38 to move the roller 35 down the passage 33, and the energized coil 84 holds the switch arm 16 inclosed position independently of pawl `1||. Consequently. the switch knob 61 can be released instantly after its operation, for the switch arm 16 will be held closed to keep the'solenoid coil 48 energized.

'I'he lswitch arm 2| carries an armature 81 arranged to be attracted by the magnetic core 88 of coil 89, which is in the circuit of battery .98, shown forfcorivenience as a separate source of current. It will be understood that in actual practice the coil 89, like the coil 48.will be connected to the usual power system of the station. A conductor 9| connects one side of battery 80 to one side of coil 89. the other side of which goes to the switch contact Il. The other dde of battery 90 is connected by conductor 82 -to a movable switch arm 93, which is normally held' (as by inherent tension) against a fixed contact 94. A wire 95 connects the contact 94 with the movable switch arm 2l.

Since the two switches 2I-22 and 93-94 are in series with coil 89, the latter remains deenergized as long as either switch is open. When theswitch arm 2I is closed by the roller 35 engaging the cam section I1' of arm |811, the coil 89 is instantly energized and holds the three connected switch arms I1-I 9-2I in actuated position even after the roller isout of engagement with the cam section I1'.

The de-energizing of the electromagnetic coils 48, 84 and 89 is done automatically when the lever 38 reaches the position 38-that is, when the roller 35 is in its lowest position 35' and is ready to return throughv passage 34. 'The lever 38 carries an insulating lug or projection 95 ar- 'ranged to strike the free end 93 of switch arm 93 away from contact 94 when the lever 3.8 is in actuated position 38'. The opening of switch 93-94 de-energizes the coil 89, whereupon 'the three switch arms I1-I9-2I instantly return to normal position. The cam-shaped end 19a of switch arm 19 is in the path of'the end 38a of lever 38, Jywhich therefore-opens the switch 19-89 simultaneously with switch `9!I,-94. It is clear, then, that the switch I'I-I8 is held 2 closed and the switch I9-2Ii held open for a predetermined period of time which is uniform in all repeated operations `of the switch mechanism without regard tothe manual release of switch knob 61. 'I'hat is to say, whether this knob is released by the operator before or after the roller 35 reaches the position 35', the switches I'l-IB and I9'-.20 remain closed and open, respectively, for the same uniform interval of time. 'I'his interval can be regulated by causing the lever 38- 1 to travel downward at a. predetermined rate of -38 to slow down its movements underthe ipu-v speed. Although the'actionv of coil 48 is sudden, a suitable retarding arrangement -(such as the well known dash-pot) can be attached to lever ence of coil-48 and spring 4I. This will be understood without further description or illustration. The opening of switch 'IS-BU-de-energizes f the two coils 48 and 84, whereupon the spring 4| actuates the lever 38 back to normal position,

thereby carrying the roller 35 up through passage 34, and' at the same time the switch arm 16 movesO-to open position. The automatic reclosing of ,switches 19-80 and 93-94 by the return 'movement of lever 38 has no eifect 0n coils 48, 84 and 89, ,because the switch arms18 switch arms `of switch mechanism W are in the position shown in Fig. 1, sothat the antenna cirl Yamplifier mechanism D also remains inoperative cuit is closed through switch '-23-24.. The circuit of the signal generator Fremains' open because of the open switch Il-I8, and the detectoron account of the open switch 21-28. `In other words, the auxiliary signalling apparatus Fand receiving apparatus D do not interfere with the regular broadcasting operation of the station.

Let us now suppose it is desired'to ascertain the number of radio receivers tuned in on the program that is being broadcast by the'station. At

`a suitable moment (as during a pause in a number or in the interval between two numbers), the person in charge of this detail turns the knob 51 and the following events take place automatically: The solenoid coil`48 is energized and pulls down the lever 38, which causes the roller 35 to close the switch I1-'-I8 and simultaneously open the adjacent switch I9-2Ii, whereby the microphone I4 is cut out and the output circuit of signal generator F is closed through coil I5. Aspreviously explained, the signal generator F operates to send| electric impulses of predetermined frequency or character through the modulating coil I5, and this special signal` is impressed upon the carrier frequency or wave of the station through the amplier-modulator M. Thus,

Y from the moment the roller 35 cams the switch arm I1 to closing position until it reaches the position 35' (an interval that need not be more than a second or two) the station is broadcasting a specialsignal, which I may call the search signal, adapted to be received `only by those radio receivers thatgare tuned for this station.

at that time. -When I describe Fig. 2, I will explain what the search signal doesv in a tuned receiver. I will now go on to describe what happens in Fig. 1 when the lever 38 reaches the `end of its forward movement. l

It will' be recalled that when the roller 35 is 'in its lowermost position 35', the actuating lever 38 is in position 38' and opens the switch arms 19v and 93, whereby thev solenoid coil 48 is deenergized and the switcharm' I1 is opened to disconnect the signal generator F. As soon as coil 48 is c'ut out of circuit, the spring 4I rocks the lever y38 clockwise and the roller 35 travels up through passage v34 to openthe switcharm 23 and close. the adjacent switch arms 25 and 21.V 'I'hehigh-frequency -transmitting generator I0 is now cut outand the antenna circuit is closed through coil I2, conductor 55 and switch 25-25, this circuit now4 being areceiving circuit tuned to thecarrier frequency of the station and adapted to receive the response signals transmitted by the tuned-in receivers. At the saine timeI the correspondingly tuned input circuit including coil 51 oi detector D' is closed through Vswitch 21-28', whereby the total electric energy received by mechanism D i'rolnthe transmitters of all tuned-in receivers ilows through the output or' load "circuit ,58 and is registered or recorded in the` instrument E, asl previously explained.

Now we l.come to Fig. 2, which shows the apparatus associated with a household radio vreceiver for automatically sending a reSDOnse to the search signal from 'tuned-in station. The radio receiver R is of any `approved make or design, having variable tuning means 96, a tuningknob 81,; and a' station. dial 91' 'I'he diagrammatic symbol 95 stands for the adjustable tuning elements. found in all 'radio receivers.

They inside ,mechanism of the receiver requires no illustration and descriptiom' since itv is a matterof lcommon knowledge in the r'adio ileld.

1. So I have indicated only the antenna coil C and the input coil 98 of the receiver mechanism. .The antenna `circuit`98 i's normally closed through switcharm |00. and contact IIII, and it will be understood that the receiver is-not operative for broadcast reception unless that switch is closed.

f 'I'he receiver cabinet contains a small radio transmitter T (preferably of the vacuum tube type). which has variable tuning means indicated diagrammatically at |02 for adjusting the transmitter to a predetermined carrier frequency ork wave length. As the construction and operation of radio transmitters are Well understood, I need not go into further details about the transmitter T except to say that it need not be of high power and long range, so that it may be a compact piece of apparatus easily housed in a radio cabinet of regularhousehold size. In some cases it may be possible to use parts of the radio reconnection between the knob 91 and the tuning elements 96 and |02. That is to say, when the mob '91 (or whatever finger piece the receiver has) is adjusted to tune the receiver for atparticular station, there is simultaneous adjustment of tuning element 96 in the receiver and of tuning element |02 inthe transmitter T. In this way the transmitter 'I' is always tuned to a carrier frequency which is dierent for each Station, and that frequency is preferably the same as that of the station tuned in. The transmitter T is normally out of operation and does not interfere with the usual functions of receiver R. In fact, the person tuning the receiver need not (and perhaps `will not) even know that transmitter T exists.

The radio receiver R also containsa signalreceiving device K, which has an input circuit IM tunable to the required frequencyr by means of a variable reactance element |05, such as one or more adjustable condensers. The driving connec.

tion |03 extends to the tuning element |05, which related to the antenna coil C of receiver R, and

the variable tuning element |05 is also part of that circuit. The output circuit of signal receiver ,K includes a coil |01 and an adjustable reactance element |08, such as a condenser, for sharply tuning this circuit to the frequency of signal generator F at the station. It should be understood that the tuning element |08 is not adjustable by means of the tuning knob 91 onthe receiver, but is adjusted once for all time to make the circuit of coil |01 resonant and responsive only to the signal of generator F. I am assuming here that this search signal is` the same for al1 stations. but

the input circuit |04 is resonant only to the carrier frequency of the particular station tuned in by the receiver R. Therefore, the signal-receiving device K is normally inoperative in its output circuit, even though the input circuit |04 isin l tune with the carrier frequency of the selected station. Not until the input coil |06 receives the signal of generator F is the output coil |01 energized to carry out its intended function, as will presently be explained. If necessary, the signal- '.1 receiver K contains a suitable current amplier between the input and output circuits, as will be understood without further description, since such amplifiers are well known. It should be noted that, although the signal receiver K is placed in receptive condition by the operator of radio receiver R for each tuned-in station, that device K is nevertheless under the exclusive control of the program tester at the broadcasting station.

The receiver R and transmitter T are electrically powered from the houselghting mains |09|09. There is, of course, the usual on-oi switch (not shown) on the radio cabinet, but the power switch connections for the transmitter T are controlled from the tuned-in station through the output coil |01 of device K. One prac-tical form of such switch control is shown in Fig. 2 by way of example. A lever pivoted at ||0 has a half-gear ||2 in mesh with a similar gear ||3 which turns about a. shaft ||4. A pulley ||5 on gear shaft ||4 carries a' cord or chain H6, which is attached to a magnetic plunger ||1 arranged to move in a coil P. When this coil is energized, the plunger ||1 is pulledl down and rocks the lever I0 down through a certain angle. A contracting coil spring H6 automatically returns the lever l I0 and all connected parts to normal position when the coil P is de-energized.

A suitable insulating panel in the radio cabinet has a two-way groove comprising a down passage il@ and an up or return passage |20. A roller |2| is mounted to travel in groove ||9|20,and is connected to the free end of levertl I0 by a link |22, which is indicated by a dotted line to prevent obscuring other parts of the drawing. The lower end of passage ||9is curved so that the roller |2| Yoccupies the position |2| at the end of its downward travel. The two spring latches |23| 24 associated with groove ||9|20 compel the roller |2| to travel down through passage ||9 and up through passage |20, as more fully explained for the similar latches 36-31 in Fig. l. A movable switch arm |25 has a central portion |25 extending across the passage |20 in the path of the returning roller |2|, so as to be actuated thereby into engagement with the. adjacent contact |26. Either a spring |21 or the inherent tension of arm |25 normally holds the latter away from contact |26, so that the members 25|26 constitute a normally open switch. An insulating rod |28 mechanically connects the switch arms |00 and |25so that they always move in unison. The spring |21 normally holds the arm |00 against its contact |0|, which also forms a stop for the action of the spring. When the roller |2| moves the switch arm |25 against the contact |26, the

switch arm |00 is at the same time moved away' from contact |0| and into engagement with the opposite contact l0|', which is connected by a Wire |29 to the output or antenna circuit of transmitter T. 'I'he power main |09 goes to contact |26, whilethe other main |09 goes directly to the transmitter. A conductor |30 connects the other side -of the transmitters power circuit to switch arm |25. The insulating panel containing groove |9| 20 may also be the support for the adjacent switches and the operating connections of roller I venience I have shown a separate storage battery |34 as the source of current for coil P, but in practice the circuit of this coil is connected to the l -houselighting -leads |09|09, or to whatever source of electric energy the radio receiver R is connected. One terminal of battery |34 goes to coil P and the other to the switch arm |32, which is normally held openv either by inherent tension or by a separate small spring. When the coil |01 is energized by the signal current of generator F, it closes the switch |32-'|33 and thus causes the solenoid coil P to be energized, whereby the switch lever is actuated as previously described.

" sharply tuned output circuit of coil |01 in device automatically adjusts the elements |02 and |05 .to tune the transmitter T and the input circuit |04 of signal receiver K to a predetermined frequency, which may be assumed to be the carrier frequency of the tuned-in station. However, this tuning of the mechanisms K and=T has no operative effect at thistimef-rst, because the K can be energized only bythe frequency or code characteristics of signal generator F, v`which is not now transmitting; and second, because the \switch |25|26 of transmitter Tis open, so

. that lthis mechanism remains inert.

Now we shall see what happens at the receiver when the search signal of generator F is transmitted from the tuned-in station during an interval when the broadcasting is silent. The signal oscillations of generator F `(imposed upon the carrier wave of the. station) pass -through the antenna coil C of the tuned-in receiver R and therefore through the coils 90 and |06, which are inductively coupled to the antenna coil. However these signal oscillations may affect'coil 98, they produce no effect in the receiver itself, which is not tuned to this special signal. The signal oscillations that enter the input circuit |04 of device K through coil |06 energize the tuned output coil |01, with the resultA that coil P is energized and operates the switch lever H0. It will be recalled that the signal generator F functions from the moment the switch arm |1 (Fig. l) is l closed by roller 35 until that roller is at` the end of its downward movement in position 35'.

, During that same interval the roller |2| (Fig. 2)

is pulled down to position |2I', so that both rollers start their return movements at the same time. Therefore, while the roller 35 holds the switches 25-26 and 21-20 closed, the roller |2| holds the switches |25-I26 and I00--IOI' closed, whereby the` transmitter T is energized to transmit radio waves at the frequency of the tuned-in station, and these waves are received by the tuned antenna coil I2 of the station, which now operates as a receiver. Thus, the tuned input ment provides for the transmission of a search signa#1 from the transmitter for a predetermined lengt of timel and that the receiver when once adjusted by the search signal-transmits its tuning indication signal for a predetermined length of time. This feature of applicants invention is veryiniportant since a device of thecharacter described is of value only if the readings obtained give a somewhat true indication of the information desired. Without the timing mechanism it canbe seen that during a reading period the number of receivers transmitting the particular signal may change due to the diierences in of the length of the signal transmitted. Accordingly, the indicator would not give a true indication of the conditions. However, when all apparatus is equipped with a timing means it will be seen that a very accurate reading may-be obtained throughout the test period due to the fact that the timing mechanisms force the transmission of the4 indicating signals throughout the test period.

The simplest kind of search signal to use for energizing the device K is one of mere frequency as distinguished from a telegraphic code signal, which would not only take longer to transmit but also require more complicated apparatus in device K. The signal of generator F may be of any frequency beyond the region of audibility and therefore outside the broadcasting range of any station. This makes the device K immune to any form of electric waves except those carrying the special signal of generator F. The operation of sending out the search signal and receiving the response signals need take only a second or so (perhaps even before the program tester lets go of the knob 61) so that the electric power `consumed by this operation is negligible. For the information of the program tester, the detector D and signal generator F may have small signal lamps |35 and |36 associated therewith and automatically lighted when those devices are functioning. While it is possible to have a separate receiving antenna for coil 51, this would be an unnecessary complication, forthe transmitting antenna of a station operates equally well as a receiving aerial with the proper switching connections. I

In Fig. 3 I have indicated a plurality of stations broadcasting simultaneously and a plurality of receivers supposed to -be tuned for different stations. If a group of receivers represented by RI is in tune for station A, and another group of receivers Rn Ais tuned for station B at the same time, the search signal sent out by station A will affect only the transmitters Tl of receivers RI. Likewise, the search signal of station B will cause only the transmitters Tn of receivers Rn to operate, and this selective operation of the transmitters takes place even if several stations should happen tosend out their search signals at the same time. Therefore the indicator or recorder E of each station is controlled only by the transmitters of those receivers that are tuned in on that station. When the same program is broadcast simultaneously byra number of stations, as in a coast-to-coasthookup, each station will receivethe response signals of the receivers tuned to that station, and-the total number of receivers listening to that program will be the s'um of the gures indicated by the response instruments E of all the stations-in the hookup. It is to be assumed that the readings of the response instruments in the subsidiary i stations are immediately relayed (either manually or automatically) to the main' station, which is thus able to give the desired information to the vsponsor of the program at the jconclusion of the broadcast. If desired, the switch arms 16 of those stationsthat are in the same hookup can be closed simultaneously by clockwork at predetermined intervals. This will be clear Without further illustration and description, since timecontrolled switches are well known.

The transmitters T are of standardized design so as to radiate the same amount of power, and after being installed they are not to be tampered with. As these small transmitters are exclusively under station control, they are in effect part of the equipment of a duly licensed station, so that the owner of a radio receiver having such a transmitter combined therewith does not become an amateur radio operator requiring a license. 'I'he signal receivers K also are of'standard design, so that the variable element always tunes the input circuit |04 to the carrier frequency of the station tuned in by the receiver R. However, once the output circuit containing coil |01 has been tuned sharply to the adopted frequency of signal generator F, that circuit is sealed against tampering. This assumes that all broadcasting stations equipped with my invention have agreed to use the same frequency for their search signal, each station transmitting that special signal on its own carrier Wave. Should certain stations decide among themselves to adopt a diiierent search signal for each station, then the output circuit of device K would have to be tuned correspondingly for each station by connecting the variable reactance element |08 with the tuning shaft |03, as previously explained for the adjustable tuning elel ments |02 and |05. It may be possible to use certain parts and circuit connections of the radio receiver R for the device K, which is really a small receiver with an output circuit tuned exclusively for the signal of generator F.

Insteadof marking the scale S of instrument E with figures representing the number of operating transmitters T (and therefore the number of listening receivers R), any other equivalent notations may be used,as for instance, numbers indicating current values -in amperes or milliamperes. The total ampere indication is then divided by a number representing the predetermined average amount of current received from each transmitter T, so that the quotientof energy from a transmitter T located near the station than from one located farv away. But it is suflicient for the practical purposes of this invention' to adopt an average value for the energy received by coil 51 from each transmitter. This average current value is readily determined by experiment and is applicable to all stations.

The procedure would be something like this:

A broadcasting station, like a newspaper, caters mainly to the inhabitants of the surrounding region. It is commonly known that radio receivers in any locality can get all the desired programs from a few stations nearby., Even in a nation-wide hookup for one program, a person will tune in on a. nearby station for the best results. So-we may fairly assume that each This means broadcasting station gets most of its listeners from a distant transmitter TI, froman intermediate transmitter T2, and from a near transmitter Tn. After making a sufficient number 0f these experiments for each group of transmitters, an average value is obtained as representing the energy received by coil 51 from the transmitter of each tuned-in receiver, and that value is used for Calibrating the instruments E. The indications thus obtained are sufiiciently close for practical purposes, for the sponsor of a radio broadcast is satisfied to know the size of his unseen audience in` round numbers.

By sending out the search signal at different points in a broadcast it is possible to get an idea of how the public is taking to the program. For

example, if the instrument E shows a greater number of tuned-in receivers at the end of a program than at the beginning or the middle, this indicates an approval or liking of the program, and a reverseshowing of the instrument would denote the publics disapproval of this program. My invention thus enables a radio advertisei` not only to know the extent of his audience but also to get up a program that will secure for him the greatest number of listenera-and that means the greatest number of possible or potential buyers of the advertised product. It is probably best to have an o'icial record of the indications of instrument E, which is therefore preferably of the recording type.

I am aware that the transmitter T, the signal receiver K, and the associated switch-operating mechanism add to the cost of the radio receiver R, of which they are a part by being embodied in the same cabinet. However, this extra cost should not be borne by the purchaser of the radio receiver but by the broadcasting advertisers, who get the practical benefits of this invention. Let us say that several broadcasting companies have agreed to adopt this program testing system and have installed the necessary equipment. These companies will then arrange withone or more radio manufacturers to put out receivers embodying the additional apparatus needed for this purpose, and the extra cost of that apparatus is borne in the first instance by the broadcasters, so that the buyers of these specially equipped receivers pay no more than, for an ordinary receiver. When a radio advertiser wants to know lhow many receivers are tuned in on his program,

the broadcaster charges him an` additional fee for this valuable information. In this Way the manufacturing cost of this invention is ultimately paid for by the radio advertisers, as seems meet and fair. f

Although I have shown and described certain specic apparatus and circuit connections, my invention is not limited to the details set forth, for its basic principles can be embodied in other ways Without departing from the scope of the appended claims.

I claim as my invention: K 1. In combination with a broadcasting station having means for transmitting at desired intervals a signal of certain frequency apart from the lbroadcast program, a radio receiver, avnormally inoperative radio transmitter associated with said receiver, said signal-transmitting means including switch mechanism for holding said means energized for a predetermined time interval, means at the receiver including an operable switch mechanism acting upon receipt of the station signal for rendering the receiver transmitter operative to transmit a characteristic signal during another predetermined time interval, means at the broadcasting station for receiving said characteristic signal, indicating means and means for applying the received characteristic signal to said indicating means.

2. The combination of a plurality of broady Acasting stations eachthereof having an electrical measuring device and means for transmitting a characteristic control signal apart from the broadcast program, operable means acting upon operation to hold the control signal transmitting meansenergized for a predetermined period of time, a plurality of radiol receivers each thereof being tunable to any one of said broadcasting stations and including means automatically settable upon receipt of the characteristic control signal from the station to which the receiver is tuned, to transmit for a certain predetermined time period a\signal characterized to indicate the receipt of said characteristic ycontrol signal, means at each of said broadcasting stations -ior receiving the indication signals sent out by those receivers which are tuned to receive signals from the broadcasting station and means at each of said broadcasting stations for energizing said measuring device in accordance with the indication signals received whereby the measuring device at any one of the broadcasting stations gives an approximate indication of the number of receivers which are tuned to that station at the time of transmission of the control signal therefrom.

ADOLPH THOMAS. 

